rustdoc: Fix sidebar heading order

[tyilo]

Makes sidebar heading order match the order of the headings in the main content.

[rustbot]

r? @lolbinarycat

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• Owners of files modified in this PR: rustdoc
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• Random selection from GuillaumeGomez, camelid, lolbinarycat, notriddle

[lolbinarycat]

please provide before/after screenshots, as well as your reasoning for why you believe the previous behavior to be incorrect.

[tyilo]

Before

Order in sidebar:

1. Provided Methods
2. Implementations on Foreign Types
3. Dyn Compatibility
4. Implementors

Order in content:

1. Provided Methods
2. Dyn Compatibility
3. Implementations on Foreign Types
4. Implementors

After

Order in sidebar & content:

1. Provided Methods
2. Dyn Compatibility
3. Implementations on Foreign Types
4. Implementors

---

Code used:

#![feature(associated_type_defaults)]

pub trait Foo {
    const REQUIRED: bool;
    const PROVIDED: bool = true;

    type REQUIRED;
    type PROVIDED = ();

    fn required() -> Self;
    fn provided() {}
}

// Foreign impl
impl Foo for () {
    const REQUIRED: bool = true;
    type REQUIRED = ();

    fn required() -> Self {
        ()
    }
}

pub struct Bar;

// Impl
impl Foo for Bar {
    const REQUIRED: bool = true;
    type REQUIRED = ();

    fn required() -> Self {
        Self
    }
}

fn main() {
    println!("Hello, world!");
}